Modelling, geophysics, Northern Ireland
| Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast. |
D M Reay
Introduction
Gravity and magnetic data can be used to model the deep structure of Northern Ireland and may be constrained by additional information from the surface geology and borehole and seismic data. From a series of 2.5D models (2D models corrected for width:length ratio) that have been constructed across Northern Ireland [1] three examples are used to illustrate the general structure of basement and sedimentary basins in Northern Ireland (P947887).
Highland Border Ridge (P947888)
Profile A trends southwest-northeast along the Highland Border Ridge which, at surface, is mostly concealed by basalt lava of the Antrim Lava Group, although Dalradian rocks crop out in the Sperrin Mountains at the southwest end and in the inlier in northeast Co. Antrim. Based on the occurrence of gravity anomaly lows it is possible to recognise the presence of two concealed sedimentary basins, the Loughguile and Kilrea-Maghera basins, the latter itself sub-divided by a basement ridge. Both basins are fault-bounded with a sediment fill up to 1000 m that thickens towards the margins. The thickening may be either syn-depositional or the result of later movement along the northeast-southwest and NNW-SSE trending marginal faults. The sedimentary fill of the Kilrea-Maghera Basin comprises Carboniferous and Triassic rocks, which are exposed near Maghera, and the Loughguile Basin contains Triassic and Cretaceous rocks, which outcrop near Armoy.
The model also demonstrates the presence of a vertical intrusion, up to 2 km wide, of highly magnetic, dense mafic rocks at Rasharkin. The gravity low near the Antrim coast has been modelled as a granitic intrusion, the concealed extension, to the northeast, of the early Caledonian Cushendun Granodiorite and associated quartz-porphyry intrusions. In the North Channel the presence of a gravity low is modelled as a narrow, c. 3 km deep, fault-bounded basin containing low density sedimentary rocks.
Foyle, Rathlin and Larne sedimentary basins (P947889)
Profile B extends from the Inishowen Peninsula in Co. Donegal, southeastwards across the Foyle and Rathlin basins towards Ballymena in Co. Antrim and then eastwards to the Larne Basin. The surface geology is dominated by basalt lava of the Palaeogene Antrim Lava Group, with Triassic to Cretaceous rocks exposed in the Foyle Basin and on the east coast of Co. Antrim. High gravity values in Co. Donegal probably indicate that the Dalradian rocks are more dense than elsewhere, although the presence of high density Lewisian basement at depth is a possibility. The gravity profile includes a bi-polar anomaly low corresponding to the Foyle and Rathlin basins, and anomaly values that decrease eastwards over the Larne Basin. The Foyle and Rathlin basins show maximum sediment thickness adjacent to the Foyle and Tow Valley faults, respectively. Densities of 2.5Mg/m3 and 2.35Mg/m3 have been adopted for the Permo-Triassic and Carboniferous rocks respectively. The abnormally low density used for the Carboniferous rocks is based on measurements from the Magilligan borehole. Although this may be appropriate for the Carboniferous in the Foyle Basin a higher density might be expected in the Rathlin Basin, resulting in a corresponding increase in maximum sediment thickness from 2.75 to c. 4 km. After crossing the Highland Border Ridge near the Rasharkin High the sedimentary rocks beneath the basalt lavas thicken to c. 1 km along strike from the outcrop of the Cross Slieve Group (see Devonian article) between Cushendun and Cushendall. The gravity high near Ballymena may be interpreted either as a slight thinning of those sedimentary rocks, or the combined gravity high and positive magnetic signature may indicate a buried mafic intrusion or lithologies similar to the Tyrone Igneous Complex (see Midland Valley Terrane article).
East of Slemish in Co. Antrim (P947872), a major fault downthrows strata to the east and about 3 km of sedimentary rocks are modelled adjacent to the Ballytober Fault. The Ballytober Fault downthrows Permo-Triassic strata by more than 800 m to the west and defines the western margin of a major horst, according to interpreted seismic data. The eastern flank of the horst is less clearly defined by a series of normal faults stepping down to the east towards the coastline of east Co. Antrim. The structure was modelled using a magnetic basement block located beneath the horst and density variations in the shallow section to improve the fit to the observed gravity anomaly. The significant gravity low east of the horst was modelled by adding thick beds of low density halite. Although halite has not been proven in this part of the basin north of the Sixmilewater Fault, it can be inferred from offshore seismic data.
Modelling of Profile B also demonstrates that the distinctive negative magnetic anomaly under Lough Foyle is best interpreted as a dyke/sill complex, rather than as an extension of the Magilligan sill or as a single major dyke. The magnetic anomaly high east of Slemish can be modelled as a normally magnetised block within the basalt lava pile, possibly reflecting an igneous centre or caldera. In contrast, the Palaeogene dolerite plug and volcanic vent of Slemish has a weak magnetic signature.
Lough Neagh Basin, Newry Igneous Complex and Mourne Mountains Central Complex (P947890)
Profile C extends from the northwest corner of Lough Neagh southeastwards to the Co. Down coast near Kilkeel. At the northwestern end the Carboniferous rocks around Draperstown are separated from the main Lough Neagh Basin by a basement ridge. This gravity high is associated with high amplitude, positive, magnetic anomalies and is likely to be an extension of the mafic rocks of the Tyrone Igneous Complex on Slieve Gallion. At the southern end of Lough Neagh, the thickness of sedimentary rocks in the Lough Neagh Basin is calculated at 2.5 km, using a density of 2.5Mg/m3. Short wavelength anomalies are modelled as variations in the thickness of near-surface, low density rocks of the Lough Neagh Group (see Late Palaeogene (Oligocene) sedimentary basins article). Southeast of Lough Neagh and beyond the southern margin of the concealed Lough Neagh Basin, Lower Palaeozoic greywackes of the Southern Uplands-Down-Longford Terrane are modelled with a lower density (2.68Mg/m3) than expected. The lower density may be caused by the presence of granitic rocks of the late Caledonian Newry Igneous Complex at depth which is modelled as a large batholith with a density of 2.65Mg/m3. The margin of the batholith is inclined northwestwards and extends for 3 km beyond the outcrop at surface, consistent with the presence of small isolated outcrops of granodiorite that occur within the greywacke. The gravity anomaly at the southeast end of the profile rises steeply to +46 mgals and reflects the presence of the large mafic igneous body beneath the low density granites of the Palaeogene Mourne Mountains Central Complex. The depth to the contact between the granite and the top of the underlying basic igneous body varies from 2.5–4 km.
3D model of the Antrim Plateau (P947891)
3D modelling can be used to predict the shape of concealed sedimentary basins and the configuration of basement beneath basalt lavas of the Antrim Plateau. An initial model of the lavas was produced which matched the observed magnetic response by varying the depth to their base, assuming constant magnetisation. The model reproduced the general features of the observed anomaly pattern although short wavelength anomalies were not resolved. The grid derived for the base of the lava pile was used as an input into the gravity model, along with a constant background field of +25mGal and a ‘basement’ layer at a depth ranging from 5–8 km. Other interfaces, in particular the base of the Lough Neagh Group and the base of Permo-Triassic strata, were allowed to vary. The model was adjusted against the observed gravity data by varying the parameters, one at a time, with a total of ten iterations. A mask was used to constrain the parts of the grid over which values were allowed to vary, to take into account the limits of the Permo-Triassic subcrop, for example.
The depth to the base of the Permo-Triassic succession from the 3D gravity model shows a good fit to borehole data, with the exception of Langford Lodge (P947841). This borehole is located on a narrow basement ridge adjacent to the outcrop of the lignite-bearing Lough Neagh Group at Crumlin, in an area of steep anomaly gradients poorly resolved by the model. It also shows the gross internal geometry of the Foyle, Rathlin, Lough Neagh and Larne basins but probably underestimates the total thickness of sedimentary rocks in the basins. This is because the lower part of the modelled Permo-Triassic succession may be replaced by a greater thickness of higher density Carboniferous strata. Detailed basin structure should not be inferred from the gravity modelling alone but from the integrated interpretation of all available geological and geophysical information, especially seismic reflection data.
This account relies heavily on the work of Richard Carruthers, who led a series of BGS projects that greatly increased our understanding of the gravity and magnetic anomalies of Northern Ireland, and who died in a tragic accident in 2001.
Reference
- ↑ Carruthers, R M, Beamish, D, Heaven, R E, Legg, I C, Mitchell, W I, Reay, D M, and Walker, A S D. 1999. Regional interpretation of gravity and aeromagnetic data from Northern Ireland. GSNI Technical Report 99/1.